Amplifier linearization using magnetically coupled feedback provided by a transformer coupled to a balun-based load

ABSTRACT

An amplifier circuit includes an amplifier having an amplifier input and an amplifier output. The amplifier circuit includes a transformer having a primary winding in series with the amplifier output and a secondary winding coupled to the amplifier input. The primary winding and the secondary winding are arranged such that a portion of a magnetic field generated by the primary winding couples to the secondary winding through a magnetically coupled feedback loop, thereby providing feedback from the amplifier output to the amplifier input. An output load arrangement is connected to the primary winding wherein the output arrangement includes a balun. The amplifier circuit may be implemented as an integrated circuit and where the primary and secondary windings are integrated in different metal layers of the integrated circuit or are otherwise arranged to effect a desired degree of magnetic coupling and feedback from the amplifier output to the amplifier input.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to and the benefit of U.S. ProvisionalPatent Application No. 63/084,497, entitled AMPLIFIER LINEARIZATIONUSING MAGNETICALLY COUPLED FEEDBACK, filed on Sep. 28, 2020, thedisclosure of which is incorporated herein by reference in its entiretyfor all purposes. This application is related to U.S. patent applicationSer. No. ______ (Attorney Docket No. SMAC-028/02US), entitled AMPLIFIERLINEARIZATION USING MAGNETICALLY COUPLED FEEDBACK, filed on even dateherewith, to U.S. patent application Ser. No. ______ (Attorney DocketNo. SMAC-028/03US), entitled DIFFERENTIAL AMPLIFIER INCLUDING DUALMAGNETICALLY COUPLED FEEDBACK LOOPS, filed on even date herewith, toU.S. patent application Ser. No. ______ (Attorney Docket No.SMAC-028/04US), entitled AMPLIFIER INCLUDING MAGNETICALLY COUPLEDFEEDBACK LOOP AND STACKED INPUT AND OUTPUT STAGES ADAPTED FOR DC CURRENTREUSE, filed on even date herewith, and to U.S. patent application Ser.No. ______ (Attorney Docket No. SMAC-028/05US), entitled METHOD OFIMPROVING LINEARITY OF AMPLIFIER CIRCUIT INCLUDING MAGNETICALLY COUPLEDFEEDBACK LOOP BY INCREASING DC BIAS CURRENT WITHOUT IMPACTING AMPLIFIERGAIN, filed on even date herewith, the disclosure of each of which isincorporated herein by reference in its entirety for all purposes.

FIELD

The present disclosure generally relates to linearization of anamplifier and, more particularly but not exclusively, to linearizationof power amplifiers operating at radio frequencies.

BACKGROUND

It is well known that the output signal from a power amplifier (PA) canbe linearized by various techniques, such as pre-distortion,feed-forward error correction and feedback. These techniques are welldescribed in both textbooks as well as in various scientific and whitepapers. However, there are drawbacks associated with each of thesetechniques.

Pre-distortion can be implemented either as analog or digitalpre-distortion. Both pre-distortion modes function by characterizing thebehavior of the PA and compensating for its response by deliberatelydistorting the signal driving the PA. If the applied distortion on theinput signal is carefully selected, the output signal from the PA couldbecome more linear.

One disadvantage of the analog form of pre-distortion is that it isdifficult to find an analog function that counteracts the behavior ofthe PA. In the digital case, the distortion components tend to imposeincreased requirements on the digital-to-analog converter (e.g. onbandwidth). Moreover, use of pre-distortion techniques often requiresinitial calibration or training. In addition, pre-distortion systems areoften sensitive to drift, e.g. in temperature.

Reference is now made to FIG. 1, which is a block diagram of anexemplary feed-forward error correction system 10 which may be used tolinearize a PA 12. In this system, the distortion generated in the PA 12is subtracted from the output signal assuming that the two delaycircuits 14, 16 match and that the two amplifiers 17, 18 are perfectlylinear. However, the system 10 is also susceptible to drift, and it isfurthermore difficult to implement delay lines with low loss.

FIG. 2 is block diagram of a so called Cartesian feedback system 20 foramplifier linearization. In the Cartesian feedback system 20, theresponse from the PA 22 is down-converted 23 and compared 24 with thebase-band IQ signals 25, 26. This approach eliminates issues with driftand detailed characterization of the transfer function of the PA 22. Onedisadvantage of the Cartesian feedback system 20 is stability and thelimited bandwidth it can handle and still maintain stable operation.Cartesian feedback is therefore not suitable for systems where thesignal bandwidth is increased to hundreds of MHz (e.g. 802.11ac wherethe largest bandwidth is 160 MHz). Cartesian feedback also adds circuitcomplexity and power dissipation.

Turning now to FIG. 3, there is shown a linear feedback system 30 foramplifier linearization. In the linear feedback system 30, a feedbacknetwork 32 is connected between the output of a PA 34 and its input.This method gives superior performance but is very difficult to getstable for all different load conditions, especially for larger signalbandwidths.

Despite these known linearization techniques, power amplifiers arecommonly implemented without feedback, where the linearity performanceinstead rely on the inherent linearity of the devices in the selectedtechnology. FIG. 4 illustrates a typical CMOS based PA implementation40, which consists of a Common Source (CS) stage (M1) followed by aCommon Gate (CG) stage (M2). The combination of a CS-stage and aCG-stage in this way is referred to as a Cascode. The linearity of aCascode is mostly dependent on the CS-stage, which typically consists ofa device with short length and lower break-down voltage for increasedgain. The CG-stage has often higher break-down voltage to allow for alarge PA supply voltage.

The CS-stage converts the input voltage into a current flowing out ofthe drain of the device M1 and the gain is dependent of thetransconductance in M1. The signal transfer can be expressed by theequation i_(dM1)=−v_(in)g_(m1), where g_(m1) is the transconductance ofM1 and v_(in) is the input voltage. The device M2 act as a currentbuffer and will feed the current from M1 to the load impedance R_(L) ifthe load inductance L_(d) resonates at the frequency of operation withthe parasitic capacitances at the drain of M2. The output voltage cantherefore be expressed as: v_(o)=−v_(in)g_(m1)R_(L). From this equationit can be understood that the device M1 must have a transconductancewhich is not dependent on the signal level, since this would result in atime invariant gain which results in distortion. If the signal is backedoff sufficiently, the linearity requirements can be fulfilled, but thisapproach will impact the efficiency of the PA. In addition, when verygood linearity is required, the device M2 will also contribute to signaldistortion.

Referring now to FIGS. 5A and 5B, there are illustrated generic voltageamplifier circuit implementations 50, 60 of the direct negative feedbackapproach to linearization shown in FIG. 3. Although such an approach ofusing of direct negative feedback would be advantageous, it will bedifficult to maintain stability when using the generic topologies ofFIGS. 5A and 5B to feedback a portion of the PA output voltage. This isbecause the loop gain to a large extent becomes dependent on the loadimpedance. With a varying load impedance, the phase shift in the loopcan therefore become larger than 180 degrees, with positive feedback andoscillation as a result.

FIG. 6 illustrates a generic transconductance amplifier system 70configured to linearize a PA 72 in a manner which avoids stabilityissues due to varying load impedance. As shown, the output current fromthe PA 72 is dependent on the input voltage and the feedback resistor R1rather than on the load impedance. However, this approach reducesoverall gain due to the presence of the feedback resistor R1.Furthermore, the CG-stage in the Cascode configuration will not belinearized.

SUMMARY

Disclosed herein are systems and methods for amplifier linearizationusing magnetically coupled feedback. Embodiments of the amplifierlinearization systems and techniques described herein may be utilized ina linear power amplifier (PA), either as a stand-alone PA, a PAintegrated in a front-end module (FEM), or as a PA integrated in anRF-ASIC.

In one aspect the disclosure relates to an amplifier circuit includingan amplifier having an amplifier input and an amplifier output. Theamplifier circuit includes a transformer having a primary winding inseries with the amplifier output and a secondary winding coupled to theamplifier input. The primary winding and the secondary winding arearranged such that a portion of a magnetic field generated by theprimary winding couples to the secondary winding through a magneticallycoupled feedback loop, thereby providing feedback from the amplifieroutput to the amplifier input. An output load arrangement is connectedto the primary winding wherein the output arrangement includes a balun.The amplifier circuit may be implemented as an integrated circuit andwhere the primary and secondary windings are integrated in differentmetal layers of the integrated circuit or are otherwise arranged toeffect a desired degree of magnetic coupling and feedback from theamplifier output to the amplifier input.

The disclosure is also directed to an amplifier circuit including anamplifier having an amplifier input and an amplifier output. Atransformer disposed to provide a signal for driving a load includes aprimary winding in series with the amplifier output. A secondary windingof the transformer is coupled to the amplifier input where the primarywinding and the secondary winding are arranged such that a portion of amagnetic field generated by the primary winding couples to the secondarywinding so as to establish a magnetically coupled feedback loop from theamplifier output to the amplifier input. A loop gain of the magneticallycoupled feedback loop is substantially independent of an impedance ofthe load and is defined at least in part by a coupling factor andturn-ratio of the transformer. The load may be included within an outputload arrangement including a balun.

In another aspect the disclosure pertains to an amplifier circuitincluding a first amplifier having a first amplifier input and a firstamplifier output. A transformer includes a first transformer componenthaving a first primary winding in series with the first amplifier outputand a first secondary winding coupled to the first amplifier input. Thefirst primary winding and the first secondary winding are arranged suchthat a portion of a first magnetic field generated by the first primarywinding couples to the first secondary winding through a firstmagnetically coupled feedback loop, thereby providing first feedbackfrom the first amplifier output to the first amplifier input. Theamplifier circuit includes a second amplifier having a second amplifierinput and a second amplifier output. The transformer further includes asecond transformer component having a second primary winding in serieswith the second amplifier output and a second secondary winding coupledto the second amplifier input, where the first primary winding and thesecond primary winding are configured to drive a load. The secondprimary winding and the second secondary winding are arranged such thata portion of a second magnetic field generated by the second primarywinding couples to the second secondary winding through a secondmagnetically coupled feedback loop, thereby providing second feedbackfrom the second amplifier output to the second amplifier input.

The disclosure is further directed to a stacked amplifier circuitincluding an input stage having first and second input portsrespectively defined by inputs of first and second transistors. Theamplifier circuit includes a transformer arrangement configured toprovide a signal for driving a load. The transformer arrangement hasfirst and second primary windings and first and second secondarywindings where a first end of the first secondary winding is connectedto an output of the first transistor of the input stage and a first endof the second secondary winding is connected to an output of the secondtransistor of the input stage. The first primary winding and the firstsecondary winding are arranged such that a portion of a first magneticfield generated by the first primary winding couples to the firstsecondary winding. The second primary winding and the second secondarywinding are arranged such that a portion of a second magnetic fieldgenerated by the second primary winding couples to the second secondarywinding. An output stage has a differential input which is AC coupled tothe first and second secondary windings and has an output connected tothe first and second primary windings. The input stage and the outputstage are arranged in a stacked configuration such that a bias currentof the output stage is reused as bias current for the input stage.

In a particular aspect the disclosure relates to a method of operatingan amplifier circuit having a transformer arranged so as to establish amagnetically coupled feedback loop between and output of an amplifierand an input of the amplifier. The method includes providing a DC biascurrent to the amplifier, and further includes increasing the DC biascurrent to improve a linearity of the amplifier circuit wherein atransfer gain of the amplifier circuit remains constant when the DC biascurrent is increased.

In yet another aspect the disclosure concerns a method of configuring anamplifier circuit having a transformer arranged so as to establish amagnetically coupled feedback loop between and output of an amplifierand an input of the amplifier. The method includes setting a loop gainof the magnetically coupled feedback loop by selecting a coupling factorand turn-ratio of the transformer.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, nature and advantages of the present disclosure willbecome more apparent from the detailed description set forth below whentaken in conjunction with the drawings in which like referencecharacters identify correspondingly throughout and wherein:

FIG. 1 depicts a block diagram of a conventional feed-forward errorcorrection system which may be used to linearize a PA.

FIG. 2 depicts a block diagram of a so called Cartesian feedback systemfor amplifier linearization.

FIG. 3 depicts a diagram of a linear feedback system for amplifierlinearization.

FIG. 4 illustrates a typical CMOS-based PA implementation.

FIGS. 5A and 5B illustrate generic voltage amplifier circuitimplementations of the direct negative feedback approach tolinearization shown in FIG. 3.

FIG. 6 illustrates a generic transconductance amplifier systemconfigured to linearize a PA in a manner which avoids stability issuesdue to varying load impedance.

FIG. 7 illustrates a functional diagram of an embodiment of an amplifierlinearization system using magnetically coupled feedback in accordancewith the disclosure.

FIG. 8 illustrates a functional diagram of an amplifier linearizationsystem which uses magnetically coupled feedback and lacks a balun at itsoutput.

FIG. 9 is a circuit-level representation of CMOS-based implementation ofan amplifier linearization system using magnetically coupled feedback inaccordance with the disclosure.

FIGS. 10A and 10B are circuit-level illustrations of a single-endedversion of a CMOS-based implementation of an amplifier linearizationsystem using magnetically coupled feedback.

FIG. 11 illustrates an implementation of a linearized power amplifierhaving stacked input and output stages.

DETAILED DESCRIPTION

Attention is now directed to FIG. 7, which illustrates a functionaldiagram of an embodiment of an amplifier linearization system 100 usingmagnetically coupled feedback in accordance with the disclosure. Thesystem 100 comprises a PA 102 having an input 104 and an output 106. Ashunt inductance (L_(in)) 108 is placed at the input 104 of theamplifier to create a parallel resonance with an input capacitance(C_(in)) of the amplifier 102. In the embodiment of FIG. 7 this parallelresonance is represented by R_(in), L_(in) and C_(in), where R_(in)represent the losses of the parallel resonance circuit. The input fromthe previous stage driving the amplifier linearization system 100 istypically a current i_(s) which generates an input voltage to theamplifier 102. An inductor 112 (L_(out)) is in series with the output106 of the amplifier 102 and laid out or otherwise arranged in such away that some of the magnetic field which it generates couples to theinductor 108. When arranged in this way the inductor 112 forms theprimary winding of a transformer which includes a secondary windingcomprised of the inductor 108.

During operation of the system 100 the magnetic field generated by thetransformer primary winding (inductor 112) couples to the secondarywinding (shunt inductance 108) through a magnetically coupled feedbackloop 116, thereby providing feedback from the amplifier output 106 tothe amplifier input 104. The amplifier 102 generates an output signal toan output load arrangement 120 connected to the transformer primarywinding (inductor 112). In one embodiment the output load arrangementincludes a balun 122 operative to convert a differential signal into asingle-ended signal driving an external load impedance Z_(L) 124.

Implementations of the amplifier system 100 are suitable for chip levelintegration where the two inductors L_(in) and L_(out) are preferablydrawn in different metal layers and laid out on top of each other. Thismagnetic coupling from the output 106 to the input 104 of the poweramplifier 102 generates direct feedback which is not believed to bepresent in state-of-the-art power amplifiers.

The amplifier system 100 can be implemented using either a single endedor differential amplifier configuration. In general it will be importantto ensure that the parasitic capacitance between the amplifier output106 and ground is small, else an excessive phase shift of up to 180degrees will result and cause instability. Small parasitic capacitanceat the amplifier output 106 is possible when integrating the poweramplifier 102 on silicon, especially when a so called SOI(Silicon-On-Isolator) technology is used, where the parasitic devicecapacitances are extremely small.

Turning now to FIG. 8, there is illustrated which a functional diagramof an amplifier linearization system 100′ using magnetically coupledfeedback in accordance with the disclosure. As shown, the linearizationsystem 100′ of FIG. 8 is substantially similar to the linearizationsystem 100 of FIG. 7 with the exception that the linearization system100′ is configured with an output load arrangement comprised solely ofan external load impedance Z_(L) 124′; that is, the output loadarrangement of the system 100′ lacks a balun. In this case the outputsignal produced by the system 100′ consists of the output voltageV_(out) across the external load impedance Z_(L) 124′.

CMOS-Based Implementation of Amplifier Linearization System

Reference is now made to FIG. 9, which is a circuit-level representationof CMOS-based implementation of an amplifier linearization system 200using magnetically coupled feedback in accordance with the disclosure.In the CMOS-based system 200 the power amplifier 202 is implemented witha Cascode stage (transistors M1 and M2). In the embodiment of FIG. 9 theinput signal is differential and represented by the current source is. Atransformer 214 having a primary winding 212 and a secondary winding 208functions as a magnetically coupled feedback loop to effect feedbackfrom an output 206 to an input 204 of amplifier 202. In one embodimentthe transformer 214 is comprised of a first transformer component 214Aand a second transformer component 214B, which may or may not beintertwined. The first transformer component 214A includes a primarywinding 212A and a secondary winding 208A and the second transformercomponent 214B includes a primary winding 212B and a secondary winding208B. An output load arrangement 220 includes a balun 222 connected tothe primary winding 212A, 212B of the transformer components 214A, 214B.The balun 222 converts the output current provided by the primarytransformer windings 212A, 212B into a single ended current, whichgenerates power upon reaching an external load impedance 224 includedwithin the output load arrangement 220.

Mathematical Characterization of Amplifier Linearization System

Attention is now directed to FIGS. 10A and 10B, which illustrate asingle-ended version of a CMOS-based implementation of an amplifierlinearization system 300 using magnetically coupled feedback. FIG. 10Adepicts components present in an exemplary physical realization of thesystem 300 while FIG. 10B further illustrates additional components towhich reference will be made in the mathematical characterization of thesystem 300 set forth below. With reference to FIG. 10B, the input signalis, as in the differential case of FIG. 9, represented by the currentsource i_(s). The capacitance C_(g) is the total parasitic capacitancefrom the input node 304 of the amplifier 302 to ground, which isdominated by the gate capacitance of M1. The inductance L₂ of thesecondary winding 308 of the transformer 314 should in combination withthe capacitance C_(g) be selected such that resonance occurs at thefrequency of operation according to Equation 1.

$\begin{matrix}{f_{0} = \frac{1}{2\pi\sqrt{L_{2}C_{g}}}} & {{Equation}\mspace{14mu} 1}\end{matrix}$

The resistance R_(p) (FIG. 10B) is not a physical resistance, instead itis a model of the losses from the resonance circuit at the input of theamplifier 302, which is primarily determined by the losses of theinductance L₂ of the secondary winding 308 of the transformer 314according Equation 2, where QL₂ is the quality factor of L₂.

R_(p)=Q_(L2)2πfL₂  Equation 2:

The device M1 converts the voltage at its gate (v_(in)) into a currentwith the transconductance (g_(m)) of M1. The inductance of the primarywinding 312 of the transformer 314 is L₁ and the so called turn-ratio ofthe transformer is defined by Equation 3.

$\begin{matrix}{n = \sqrt{\frac{L_{2}}{L_{1}}}} & {{Equation}\mspace{14mu} 3}\end{matrix}$

The transformer 314 may also be characterized by its so-called couplingfactor k, which is a measure of the amount of inductive coupling thatexists between the two coils 308, 312 and is expressed as a fractionalnumber between 0 and 1, where 0 indicates zero or no inductive coupling,and 1 indicating full or maximum inductive coupling.

The load inductance L₀ and capacitance C_(o) are chosen such thatresonance occurs at the frequency of operation according Equation 4. TheAC coupling capacitance C_(ac) is selected such that its impedance islow compared to the load impedance R_(L). Hence, the load seen by theoutput current i_(o) can, at the frequency of operation, be representedby the load impedance R_(L).

$\begin{matrix}{f_{0} = \frac{1}{2\pi\sqrt{L_{o}C_{o}}}} & {{Equation}\mspace{14mu} 4}\end{matrix}$

With the definitions made above, it is possible to calculate the currentgain, which results in Equation 5.

$\begin{matrix}{A_{i} = {\frac{i_{o}}{i_{s}} = {A_{i\infty}\frac{A\beta}{1 - {A\beta}}}}} & {{Equation}\mspace{14mu} 5}\end{matrix}$

Where Aβ is the loopgain of the feedback loop as defined in Equation 6and A_(i∞) is the current gain with infinite loop gain as defined inEquation 7.

$\begin{matrix}{{A\beta} = {{- g_{m}}R_{p}\frac{k}{n}}} & {{Equation}\mspace{14mu} 6} \\{A_{i\infty} = \frac{n}{k}} & {{Equation}\mspace{14mu} 7}\end{matrix}$

As may be appreciated from Equation 7, when the loop gain Aβ is infinitethe gain of the amplifier system 300 depends only on the characteristicsof the transformer 314 as expressed by A_(i∞). This means that underthese conditions the gain is substantially independent of the inherentgain characteristics of the amplifier 302, i.e. it is substantiallyindependent of g_(m). This brings significant advantages as g_(m) is tolarge extent dependent on temperature, bias current, silicon technologybehavior, loading effects, and supply voltage variations. In practice,it is of course not possible to realize an infinite loop gain. However,the inventor has found that when the loop gain is on the order of 10 dBthe amplifier system 300 generally exhibits the desired behavior.

Key Features of Amplifier Linearization System

From the above equations various features and advantages of the presentamplifier linearization system are apparent. First, one key advantage ofthe present system is that the load impedance is not found in any of thepreceding expressions. Hence, stability is much easier to accomplish inthe present system relative to conventional power amplifier systemssince the loop gain is independent on the load impedance; that is, anyvariation of the load impedance in the present system will not impactloop stability. Second, another important aspect of the present systemis that the gain is defined by the coupling factor and the turn-ratio ofthe transformer, which are constant across both temperature and signallevel variations. In a Cascode amplifier without this feedback, the gainis achieved by converting the input voltage of the Cascode into currentby the transconductance of the Common Source stage of the Cascode, whichis both temperature as well as signal level dependent. A gain which isdependent on the signal level results in distortion, which is typicallysolved by backing off the signal level from the compression point of theamplifier. However, the presently disclosed feedback topology enables anamplifier to operate closer to compression without degrading itslinearity performance. Yet another key advantage of the present systemis that its input impedance is low due to the magnetically coupledfeedback, which results in a lower voltage swing at the input of theCascode. Since the input current is generated by a driver stage, lowvoltage swing at the driver output reduces its distortion and makes theoverall system more linear.

Additional Novel Features and Advantages

As can be seen in FIGS. 9, 10A and 10B, assuming the presence of a shuntinductance at the input of the amplifier 302 (FIGS. 10A, 10B), themagnetically coupled feedback network does not require any additionalcircuitry other than the series inductance 312 located at the output ofthe amplifier 302. Furthermore, no additional DC current is required tobias the magnetically coupled feedback network.

From Equation 6, it can be seen that the loop gain is proportional tothe transconductance of the Common Source stage, which offers importantadvantages. For example, since g_(m) is dependent on the current flowingin the Common Source stage, the loop gain will increase when the signalis increased, i.e., the loop gain will become large when the signallevel is large. This is a preferred characteristic from a linearitypoint of view.

It is known that state-of-the-art power amplifiers often operate in socalled class AB operation, which means that DC power dissipationincreases with the signal level. Due to the magnetically coupledfeedback topology described herein, the amplifier can be biased in classA operation, which means that the DC power dissipation remains constantindependent on the signal level. In many applications this is a veryimportant feature. One such application is in WiFi power amplifiers,where a high output power is typically transmitted when the link isestablished. In accordance with the present disclosure, a high outputpower can be transmitted without increasing the power dissipation.

It is also observed that the signal transfer function of the presentlinearized power amplifier system is defined by passive layoutstructures (i.e., one or more transformers) which are independent ofsignal level and temperature. This feature can be used to program thepower amplifier system for different use cases. For example, when verygood linearity is required the DC bias current can be increased toprovide improved linearity without impacting the transfer gain of theamplifier system. In contrast, changing the bias conditions ofstate-of-the-art amplifiers will also impact the gain of suchamplifiers.

It is further observed that the load impedance has little or no directimpact on loop stability. In addition, the input impedance of the outputstage is low, which reduces the voltage swing at the input. Finally, thedisclosed feedback concept is suitable for chip-integration, both inCMOS and Bipolar technologies.

Stacked Input and Output Stages

Turning now to FIG. 11, an implementation is shown of a linearized poweramplifier system 600 having an input stage 610, an output stage 620, atransformer arrangement 630 and an output balun 640. The input stage 610and the output stage 620 are realized in different integrated circuitlayers and are laid out so as to be stacked on each other. Thetransformer arrangement 630 effects magnetically coupled feedback fromfirst and second outputs 644A, 644B of the output stage 620 to an input648 of the output stage 630. As shown, the output stage 620 has adifferential input 652A, 652B which is AC coupled 656A, 656B to firstand second secondary windings 662A, 662B of the transformer arrangement630. One key advantage of this stacked configuration is that the biascurrent of the output stage 620 is reused as bias current for the inputstage 610, which improves efficiency of the power amplifier system 600.

Where methods described above indicate certain events occurring incertain order, the ordering of certain events may be modified.Additionally, certain of the events may be performed concurrently in aparallel process when possible, as well as performed sequentially asdescribed above. Accordingly, the specification is intended to embraceall such modifications and variations of the disclosed embodiments thatfall within the spirit and scope of the appended claims.

The foregoing description, for purposes of explanation, used specificnomenclature to provide a thorough understanding of the claimed systemsand methods. However, it will be apparent to one skilled in the art thatspecific details are not required in order to practice the systems andmethods described herein. Thus, the foregoing descriptions of specificembodiments of the described systems and methods are presented forpurposes of illustration and description. They are not intended to beexhaustive or to limit the claims to the precise forms disclosed;obviously, many modifications and variations are possible in view of theabove teachings. The embodiments were chosen and described in order tobest explain the principles of the described systems and methods andtheir practical applications, they thereby enable others skilled in theart to best utilize the described systems and methods and variousembodiments with various modifications as are suited to the particularuse contemplated. It is intended that the following claims and theirequivalents define the scope of the systems and methods describedherein.

Also, various inventive concepts may be embodied as one or more methods,of which an example has been provided. The acts performed as part of themethod may be ordered in any suitable way. Accordingly, embodiments maybe constructed in which acts are performed in an order different thanillustrated, which may include performing some acts simultaneously, eventhough shown as sequential acts in illustrative embodiments.

All definitions, as defined and used herein, should be understood tocontrol over dictionary definitions, definitions in documentsincorporated by reference, and/or ordinary meanings of the definedterms.

The indefinite articles “a” and “an,” as used herein in thespecification and in the claims, unless clearly indicated to thecontrary, should be understood to mean “at least one.”

The phrase “and/or,” as used herein in the specification and in theclaims, should be understood to mean “either or both” of the elements soconjoined, i.e., elements that are conjunctively present in some casesand disjunctively present in other cases. Multiple elements listed with“and/or” should be construed in the same fashion, i.e., “one or more” ofthe elements so conjoined. Other elements may optionally be presentother than the elements specifically identified by the “and/or” clause,whether related or unrelated to those elements specifically identified.Thus, as a non-limiting example, a reference to “A and/or B”, when usedin conjunction with open-ended language such as “comprising” can refer,in one embodiment, to A only (optionally including elements other thanB); in another embodiment, to B only (optionally including elementsother than A); in yet another embodiment, to both A and B (optionallyincluding other elements); etc.

As used herein in the specification and in the claims, “or” should beunderstood to have the same meaning as “and/or” as defined above. Forexample, when separating items in a list, “or” or “and/or” shall beinterpreted as being inclusive, i.e., the inclusion of at least one, butalso including more than one, of a number or list of elements, and,optionally, additional unlisted items. Only terms clearly indicated tothe contrary, such as “only one of” or “exactly one of,” or, when usedin the claims, “consisting of,” will refer to the inclusion of exactlyone element of a number or list of elements. In general, the term “or”as used herein shall only be interpreted as indicating exclusivealternatives (i.e. “one or the other but not both”) when preceded byterms of exclusivity, such as “either,” “one of,” “only one of,” or“exactly one of.” “Consisting essentially of,” when used in the claims,shall have its ordinary meaning as used in the field of patent law.

As used herein in the specification and in the claims, the phrase “atleast one,” in reference to a list of one or more elements, should beunderstood to mean at least one element selected from any one or more ofthe elements in the list of elements, but not necessarily including atleast one of each and every element specifically listed within the listof elements and not excluding any combinations of elements in the listof elements. This definition also allows that elements may optionally bepresent other than the elements specifically identified within the listof elements to which the phrase “at least one” refers, whether relatedor unrelated to those elements specifically identified. Thus, as anon-limiting example, “at least one of A and B” (or, equivalently, “atleast one of A or B,” or, equivalently “at least one of A and/or B”) canrefer, in one embodiment, to at least one, optionally including morethan one, A, with no B present (and optionally including elements otherthan B); in another embodiment, to at least one, optionally includingmore than one, B, with no A present (and optionally including elementsother than A); in yet another embodiment, to at least one, optionallyincluding more than one, A, and at least one, optionally including morethan one, B (and optionally including other elements); etc.

In the claims, as well as in the specification above, all transitionalphrases such as “comprising,” “including,” “carrying,” “having,”“containing,” “involving,” “holding,” “composed of,” and the like are tobe understood to be open-ended, i.e., to mean including but not limitedto. Only the transitional phrases “consisting of” and “consistingessentially of” shall be closed or semi-closed transitional phrases,respectively, as set forth in the United States Patent Office Manual ofPatent Examining Procedures, Section 2111.03.

What is claimed is:
 1. An amplifier circuit, comprising: an amplifierhaving an amplifier input and an amplifier output; a transformer havinga primary winding in series with the amplifier output and a secondarywinding coupled to the amplifier input wherein the primary winding andthe secondary winding are arranged such that a portion of a magneticfield generated by the primary winding couples to the secondary windingthrough a magnetically coupled feedback loop, thereby providing feedbackfrom the amplifier output to the amplifier input; and an output loadarrangement connected to the primary winding wherein the output loadarrangement includes a balun.
 2. The amplifier circuit of claim 1wherein the amplifier circuit is implemented as an integrated circuitand wherein the primary and secondary windings are integrated indifferent metal layers of the integrated circuit.
 3. The amplifiercircuit of claim 1 wherein the amplifier is implemented in a cascodeconfiguration.
 4. The amplifier circuit of claim 1 wherein an inductance(L₂) of the secondary winding is selected in conjunction with aparasitic capacitance (C_(g)) between the amplifier input and groundsuch that resonance of the amplifier circuit occurs at a resonantfrequency (f₀) equivalent to a desired frequency of operation.
 5. Theamplifier circuit of claim 4 wherein f₀ is represented as:$f_{0} = {\frac{1}{2\pi\sqrt{L_{2}C_{g}}}.}$
 6. The amplifier circuit ofclaim 1 wherein the amplifier circuit is characterized by a current gainA_(i) of:$A_{i} = {\frac{i_{o}}{i_{s}} = {A_{i\infty}\frac{A\beta}{1 - {A\beta}}}}$where i_(s) is the current provided to the amplifier input, i_(o) is thecurrent provided to the output load arrangement, and Aβ is a loop gaindefined by: ${A\beta} = {{- g_{m}}R_{p}\frac{k}{n}}$ where here g_(m) isthe transconductance of the amplifier, k is a coupling factor betweenthe primary winding and the secondary winding, R_(p) is a resistance toground at the amplifier input, where A_(i∞) is the current gain withinfinite loop gain and is expressed as: $A_{i\infty} = \frac{n}{k}$where n is a turn-ratio of the transformer, which can be expressed as:$n = \sqrt{\frac{L_{2}}{L_{1}}}$ and where L₁ is an inductance of theprimary winding and L₂ is an inductance of the secondary winding.
 7. Theamplifier circuit of claim 1 wherein the output load arrangement furtherincludes a load element connected to the balun.
 8. The amplifier circuitof claim 1 wherein the amplifier circuit is implemented as an integratedcircuit and wherein the primary and secondary windings are integrated ina single metal layer of the integrated circuit.
 9. The amplifier circuitof claim 1 wherein the primary winding has a first end connected to theamplifier output and a second end connected to a first end of the balun.10. An amplifier circuit, comprising: an amplifier having an amplifierinput and an amplifier output; a transformer configured to providefeedback from the amplifier output to the amplifier input through amagnetically coupled feedback loop; and an output load arrangementconnected to the transformer wherein the output load arrangementincludes an inductive element.
 11. The amplifier circuit of claim 10wherein the output load arrangement further includes a load elementconnected to the balun.
 12. The amplifier circuit of claim 10 whereinthe transformer includes a first inductor connected in series to theamplifier output and a second inductor connected in shunt to theamplifier input.
 13. The amplifier circuit of claim 10 wherein a loopgain of the magnetically coupled feedback loop is substantiallyindependent of an impedance of the output load arrangement and isdefined at least in part by a coupling factor and turn-ratio of thetransformer.
 14. The amplifier circuit of claim 10 wherein a loop gainof the magnetically coupled feedback loop automatically increases inresponse to corresponding increases in a level of an input signalapplied to the amplifier input so as to maintain a substantiallyconstant level of an output signal produced at the amplifier output. 15.The amplifier circuit of claim 10 wherein power dissipation of theamplifier remains substantially constant independent of characteristicsof an input signal applied to the amplifier input.
 16. The amplifiercircuit of claim 10 wherein a current gain of the amplifier circuit issubstantially independent of gain characteristics of the amplifier whenthe transformer is configured such that a loop gain of the magneticallycoupled feedback loop is greater than 10 dB.
 17. The amplifier circuitof claim 1 wherein a loop gain of the magnetically coupled feedback loopis independent of an impedance of the output load arrangement and isdefined at least in part by a coupling factor and turn-ratio of thetransformer.
 18. The amplifier circuit of claim 1 wherein a loop gain ofthe magnetically coupled feedback loop automatically increases inresponse to corresponding increases in a level of an input signalapplied to the amplifier input so as to maintain a substantiallyconstant level of an output signal produced at the amplifier output. 19.The amplifier circuit of claim 1 wherein power dissipation of theamplifier remains substantially constant independent of characteristicsof an input signal applied to the amplifier input.
 20. The amplifiercircuit of claim 1 wherein a current gain of the amplifier circuit issubstantially independent of gain characteristics of the amplifier whenthe transformer is configured such that a loop gain of the magneticallycoupled feedback loop is greater than 10 dB.
 21. The amplifier circuitof claim 15 wherein the characteristics include at least one ofamplitude and power.
 22. The amplifier circuit of claim 15 wherein thecharacteristics include at least one of modulation type and bandwidth.23. The amplifier circuit of claim 19 wherein the characteristicsinclude at least one of amplitude and power.
 24. The amplifier circuitof claim 19 wherein the characteristics include at least one ofmodulation type and bandwidth.
 25. The amplifier circuit of claim 1wherein power dissipation of the amplifier remains substantiallyconstant independent of a level of an output signal produced at theamplifier output.
 26. The amplifier circuit of claim 10 wherein powerdissipation of the amplifier remains substantially constant independentof a level of an output signal produced at the amplifier output.
 27. Theamplifier circuit of claim 1 wherein the transformer is configured suchthat a degree of coupling between the primary winding and the secondarywinding is selected based upon a target current gain of the amplifiercircuit.